One easy retrofit solution (though certainly not anywhere as secure as Lexan) is 3m Window Film. This will probably be good enough to keep flashbangs from being fired thru windows. http://solutions.3m.com/wps/portal/3M/en_US/Window_Film/Solu...

The ad copy on that page reads:

   Because these films help hold the glass together,
   a stronger barrier is created, slowing down
   criminals to the point where they may choose
   to flee for an easier target.

Sadly, there's often only a subtle difference between a violent criminal and a SWAT team member. The personality types are often quite similar.